Power semiconductor devices are used extensively in power converters to raise or lower a DC voltage to another level (buck or boost), convert AC power to DC power (rectification), and convert DC power to AC power (inversion). They are also increasingly being used in electrical circuit breakers (solid-state circuit breakers) to control the distribution of electrical power, protect electrical loads, and isolate faults and other unacceptable overload conditions. Power semiconductors are capable of switching OFF in a matter of microseconds. In contrast, air-gap circuit breakers typically take several milliseconds to respond to and isolate a fault. The fast reaction time offered by solid-state circuit breakers is highly desirable since it can prevent damage to line equipment and electrical loads when a fault occurs, reduce the risk of electrical fires, and prevent arc flashes from occurring.
Although the fast reaction capability of power semiconductor devices is desirable, various problems arise in their deployment in circuit breakers. These problems relate to differences in how the power semiconductor devices are controlled to operate in solid-state circuit breakers, compared to how they are controlled to operate in power converters, and the manner by which power semiconductor devices are typically packaged.
Power semiconductor devices in power converters are often controlled to operate as switches (i.e., in “switch-mode”) and switching is coordinated among the various power semiconductor devices that make up the converter so that conduction losses (i2R losses) and switching losses are minimized. In contrast, in solid-state circuit breakers the power semiconductors remain ON during normal operating conditions and are switched OFF only when a fault or other unacceptable overload condition occurs. This application-dependent difference in how the power semiconductor devices are controlled to operate results in different heat generating mechanisms and heat transfer requirements.
So that power semiconductor devices are not damaged or destroyed due to excessive heat and are able to continue operating at a temperature below the maximum permissible operating temperature specified by the manufacturer, it is imperative that the heat generated by the power semiconductor devices be transferred away as efficaciously as possible. The ability to transfer heat away from power semiconductor devices is determined in large part by their packaging. Unfortunately, the packaging used for power semiconductor devices is normally specifically designed and constructed for power converter applications and is inadequate to satisfy the heat transfer requirements needed for circuit breaker applications.
Conventional power semiconductor device packaging is also mechanically vulnerable when used in a solid-state circuit breaker since the power semiconductor devices must be able to withstand significantly higher currents, particularly when a short circuit occurs in the load circuit that the circuit breaker is serving to protect. Although the power semiconductor device in a solid-state circuit breaker is switched OFF soon after the short circuit occurs, there is a short period of time that it remains ON until it can be switched OFF. During this time, the power semiconductor device and packaging must pass and withstand thousands and even tens of thousands of amps.
Unfortunately, conventional power semiconductor device packaging is not normally able to withstand these high currents and heat that is generated. The high currents and extreme thermal cycling also leads to die-substrate tension, which can cause physical separation of the power semiconductor die from the packaging substrate, and de-bonding of conductive layers from the packaging substrate. Moreover, since conventional power semiconductor packaging typically uses bonding wires to connect the power semiconductor die pads to conductive traces on the packaging substrate, the significantly higher currents and extreme thermal cycling causes bonding wire fatigue and wire-bond-related package failure.